A steel material used for automobiles, structural materials, and the like is required to be excellent in such mechanical properties as strength, workability, and toughness. In order to improve these mechanical properties comprehensively, it is effective to refine the structure of the steel material. To this end, a number of manufacturing methods for obtaining a steel material with a fine-grained structure have been sought. Further, by refining the structure, it is possible to obtain a high-strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloy elements added is reduced.
As a method for refining the structure of a steel material, it is known that a large rolling reduction is carried out especially in the later stage of hot finish rolling to refine austenite grains; and to increase rolling strains in a steel sheet, thereby obtaining fine ferrite grains after rolling. Further, in view of inhibiting recrystallization and recovery of the austenite grains and facilitating the ferrite transformation, it is effective to cool the steel sheet to a temperature from 600° C. to 750° C. as quickly as possible after rolling. That is, subsequent to hot finish rolling, it is effective to arrange a cooling device capable of cooling more quickly than ever before to thereby rapidly cool the steel sheet after the rolling. And in rapidly cooling the post-rolled steel sheet in this way, it is effective to increase a volume of cooling water per unit area sprayed over the steel sheet, that is, to increase a water flow density in order to enhance a cooling capability.
On the other hand, not only is it necessary to simply perform rapid cooling in this way, it is also required to accurately stop cooling so as to obtain a required metal structure; and to control a temperature of a steel sheet at a time of stopping the rapid cooling, to a predetermined temperature. Thereby, a desired steel sheet structure can be obtained and the quality of a large number of steel sheets manufactured can be stabilized.
Here, the temperature at a time of stopping rapid cooling is hereinafter referred to as a rapid-cooling stopping temperature. The rapid-cooling stopping temperature is described below in more detail. A temperature distribution in a thickness direction of a steel sheet during rapid cooling is in a transient state where the heat on the surface layer area is rapidly deprived due to the rapid cooling and the surface temperature is lower than the central temperature. When the rapid cooling is stopped in such a state, the heat in the central area is diffused toward the surface layer area, as time passes, to become uniform. The rapid-cooling stopping temperature refers to a temperature of a steel sheet in this uniform state; and is almost equivalent to a value obtained by measuring a surface temperature of a steel sheet with a radiation thermometer after a certain amount of time passes from the time when the rapid cooling has been stopped.
Patent Document 1 discloses a manufacturing method of a hot-rolled steel sheet characterized in that: when changing, during hot rolling, to other hot-rolling conditions different from prescribed hot-rolling conditions, and continuing hot rolling, the values of cooling conditions set for a water-cooling device, which values enable a coiling temperature of a steel sheet to become a target value, are determined based on these other hot-rolling conditions and on a measured value of a temperature of the steel sheet on an entry side of the water-cooling device; and further the set values of the cooling conditions of the water-cooling device are corrected and reset based on these other hot-rolling conditions and on the measured value of the temperature of the steel sheet on the entry side of the water-cooling device. According to this, the temperature of the steel sheet after rolling can be controlled to a target temperature.
Thus, Patent Document 1 suggests a cooling method comprising arranging a rapid-cooling device on an exit side of a hot finishing mill, wherein a thermometer is disposed between the finishing mill and the rapid-cooling device.